For deep space communication, optical frequencies provide many advantages over presently used technologies. Higher data rates, less power and mass, and smaller beam divergence are some of the benefits provided by laser communications. Current space communication links incorporate either a ground-based station or an earth-orbiting spacecraft to send to and receive optical signals from mission spacecraft. Design considerations require the transmitter on the spacecraft to consist of a pulse position M-ary (PPM) modulated, frequency doubled, Nd:YAG laser operating a 0.532 .mu.m. The anticipated range of communications rates for deep space is between 3 kbits/sec and 50 Mbits/sec (dependent on the range of the mission). Assuming M=256, this corresponds to nominal laser repetition rates between 380 Hz and 6.3 MHz. To achieve these rates of modulation, Q switching is utilized at the lower rates, and cavity dumping is utilized for the higher rates.
In laser Q-switching, lasing is held off by introducing loss into the resonator cavity while energy is pumped into and stored in the atomic population inversion. Once the desired inversion is attained, cavity losses are reduced to allow lasing. In this mode, it is possible to attain a single large pulse output from the laser. The frequency range of Q switching extends up to a range of 50 to 100 kHz, with no lower boundary. The upper repetition rate is limited by the finite time required to repump the inversion in the gain medium of the laser. To extend the upper repetition rate further, a cavity dumping technique must be used. In cavity dumping, energy is stored in the photon field of the laser instead of the atomic inversion. The photon field is generated between two mirrors of maximum reflectivity. To extract a pulse from the resonator, the beam is electro-optically or acousto-optically deflected out of the cavity between the two mirrors.
Repetition rates achievable with cavity dumping have been demonstrated between 125 kHz and 10 MHz. See W. Koechner, Solid-State Laser Engineering, New York: SpringerVerlag, 1976, pp. 444-446. The lower limit is reached when the photon field within the resonator is reduced to one photon after dumping the field. At this point the beginning of the build up is dependent on the statistical variance of spontaneous emission. Hence, if the cavity is dumped of all its energy, cavity dumping becomes unstable. If the cavity is not dumped of all its energy, for example by inducing an incomplete polarization flip with an electro-optic modulator, this lower limit can be extended. The upper rate of cavity dumping is limited by the switching time of the modulator. To extend pulse rates beyond 10 MHz, a mode locked laser must be used.
Additionally, frequency doubling of the laser radiation is often desired for efficient detection at the receiver. The frequency-doubling conversion efficiency is a function of the intensity in the nonlinear doubling crystal. As the intensity increases, the conversion efficiency also increases. Therefore, to maximize efficiency, intra-cavity doubling is desirable because photon flux levels are much higher inside the laser resonator.
Techniques for intra-cavity doubling of Q switched lasers are well known. However, intra-cavity frequency doubling/cavity dumping is less desirable since placing a frequency-doubling crystal in the primary cavity causes losses, thus reducing the stored energy in the laser resonator. Once the energy is frequency doubled it can no longer stimulate emission in the laser gain medium, and therefore will not experience gain in the laser resonator. On the other hand, in external frequency doubling, the fundamental wavelength that is undoubled is lost resulting in a lower conversion efficiency. It would be desirable to have external frequency doubling with cavity dumping while feeding back into the cavity undoubled frequency light for greater efficiency.